Physiology of the gut and mechanisms of prebiotic effect

Physiology of the gut and mechanisms of prebiotic effect

Joanne Slavin, Ph.D, R.D.Department of Food Science and Nutrition

University of Minnesota

Fermentable carbohydrate: GI Tract Fermentable carbohydrate: GI Tract

• Incompletely digested and partially absorbed in small intestine– Bonds resistant to digestive

enzymes• Fermented by bacteria in

large intestine– SCFAs, H2 , CH4 , CO2

• And/or excreted in feces

• Lactose and sucrose, on occasion

• Dietary fiber• Functional fiber• Oligosaccharides• Resistant starch• Sugar alcohols/rare

sugars

Fermentable CarbohydratesFermentable Carbohydrates

• Some celluloses and hemicelluloses• Resistant maltodextrins• Lignin (polyphenolic compound)

Non-fermentable/Poorly Fermentable Carbohydrates (and Lignin) Non-fermentable/Poorly Fermentable Carbohydrates (and Lignin)

Fiber balance studiesFiber balance studies

• Must feed defined, controlled diet for a long enough time for adaptation– Wheat bran (56%)– Oat bran (96%)– Polydextrose (50%)– Pectin (100%)– Psyllium (25 – 50%)

Fiber fermentation measuresFiber fermentation measures

• Intubation studies – considered gold standard – very invasive

• Ileostomy model – different GI tract• Hydrogen breath test – does not relate

to symptoms• SCFAs in venous blood or in feces• Fiber in fecal samples• Fecal microflora

Fate of fiber in the gutFate of fiber in the gut

• Fiber digestibility ranges from 2 – 100%• Little Solka Floc (purified cellulose) is

degraded during transit• Difficult to measure fiber disappearance

since microbial cell wall in feces inflate values

• Slavin et al. J Nutr 1981;111:287-297.

Can breath hydrogen and methane predict fiber digestibility? Can breath hydrogen and methane predict fiber digestibility?

• Breath hydrogen and methane and fecal fiber were measured in human subjects consuming 0, 30, and 60 g soy fiber

• No relationship was found between breath gases and fiber digestion

• Breath gases were not changed with different fiber intakes

• McNamara et al. Am J Clin Nutr 1986;43:898- 902.

Short-chain Fatty AcidsShort-chain Fatty Acids

• Produced by bacterial fermentation of fiber in colon

• Physiological effects– Source of energy– Lowers colonic pH

• Inhibits pathogenic bacteria growth– Butyrate promotes colon health– Propionate may lower cholesterol

Acetate Propionate

Butyrate

Are SCFAs and fiber digestibility related in vivo? Are SCFAs and fiber digestibility related in vivo?

• Measured fiber digestibility and colonic SCFAs (collected in dialysis bags in feces) on wheat bran and vegetable diets

• WB fiber more digestible – higher production of butyrate and propionate

• No correlation between fiber digestibility and SCFA concentrations

• Fredstrom et al. JPEN 1994;18:14-19.

In vitro fermentation methodIn vitro fermentation method

• Mimics colonic conditions in a closed laboratory system• 0.5g fiber added to each 100mL serum bottle

– Negative control = no fiber– Positive control = glucose

• Fecal samples obtained from 3 donors to provide representative colonic microflora

• Fecal slurry made to imitate colonic conditions• Oxyrase® oxygen-reducing enzyme added to

reducing solution• Resazurin added to confirm anaerobic conditions• Serum bottles flushed with CO2 to remove oxygen

In vitro fermentation methodIn vitro fermentation method

• Bottles incubated in shaking water bath at 37°C

• Fermentation stopped at 0, 4, 8, 12, and 24 hours, one bottle was removed for each fiber

• Duplicate samples removed from each bottle

• Samples analyzed by GC/MS for short- chain fatty acid content

In vitro fermentation - pros and cons In vitro fermentation - pros and cons

• Pros– Time-efficient– Non-invasive

• Cons– Static system– Slurry components may not adequately

represent colon

Short chain FOS exhibit more rapid fermentation than long-chain inulin Short chain FOS exhibit more rapid fermentation than long-chain inulin

• 3 FOS and 3 inulin (IN) compared• The rate of FOS fermentation was higher than

IN from 0-4 hours and rate of IN fermentation was higher than FOS from 12 – 24 hours

• The longest chain inulin produced the lowest amount of butyrate at 24 hours

• Stewart et al. Nutr Res 2008;28:329-334.

Particle size and fraction of wheat bran influence SCFAs Particle size and fraction of wheat bran influence SCFAs

• Compared particle size and fraction of wheat bran and SCFA production

• Greater SCFA production with small particle size bran

• Fine by-product produced most SCFAs• Molar percentage of butyrate at 24 hours

greater for large particle size bran• Stewart &Slavin. Br J Nutr 2009 102:1404-7

What is gut health?What is gut health?

• Normal bowel function – subjective • Regular bowel movements• Desirable bowel movements – not

diarrhea or constipation• Absence of symptoms – bloating, gas,

noises, pain• Quality of life suffers greatly when gut

health is compromised

Common gut health issuesCommon gut health issues

• Constipation – how easily and how regularly you have a bowel movement– Low fiber diet– Sedentary lifestyle– Resisting the urge to have a bowel movement– Stress– Long-term laxative use– Some medications

Exit strategy – how to stay regular Exit strategy – how to stay regular

• 15% adults report chronic constipation• More common in older people• At least twice as common in women• Spend $750 million on laxatives and

irregularity leads to 8 million trips to the doctor and $7 billion for evaluation and diagnostic testing

• Nutrition Action Newsletter, March 2009

Irritable bowel syndrome (IBS)Irritable bowel syndrome (IBS)

• Complex disorder of lower intestinal tract – accounting for 20-50% of referrals to GI clinics, majority being women 20-40 years old

• Mixed support for dietary fiber, probiotics, or prebiotics in treatment of IBS

• Conservative approach best - mixed fibers• Williams & Slavin. Topics in Clin Nutr

2009;24:262-271

Defining gut healthDefining gut health

• Less than 3 stools per week or more than 3 stools per day – considered abnormal

• Stool weight greater than 200 g/day is clinically defined as diarrhea, but many vegetarians have stool weights of 300 g/day or more

• Stool consistency important to consumers – requires presence of water-insoluble fecal solids such as non-digested dietary fiber or bacterial cell walls

• Gastroenterology 1999;116:1464

The Scoop on PoopThe Scoop on Poop

• Feces are about 75% water – highly variable – much higher with diarrhea - of the remaining feces, about 1/3 is dead bacteria, 1/3 is undigested carbohydrate, and 1/3 is protein, fat, mucus, dead cells, and inorganic material

• Smell results from products of bacterial action – sulfur or nitrogen-rich compounds such as indole, skatole, and mercaptans, and the inorganic gas hydrogen sulfide

Normal digestion and absorptionNormal digestion and absorption

• Typically about 95% of macronutrients (carbohydrates, fats, proteins) get digested and absorbed

• Leftovers of digestion and absorption (feces) include fiber, bacteria, water

• Making nutrients less digestible (low digestible carbohydrates – starch, sugar alcohol, etc) – may lower calories but may create GI issues – intestinal gas, diarrhea, etc

Non-dietary factors that affect laxation Non-dietary factors that affect laxation

• Stress• Exercise (Oettle. Gut 1991;32:941)• Smoking• Coffee drinking• Drugs (laxatives) (Lembo A, Camilleri

M. New Eng J Med 2003;349:1360)• Personality (Tucker et al.

Gastroenterology 1981;81:879)

Nutrition and lifestyle and bowel movement frequency Nutrition and lifestyle and bowel movement frequency

• 20,630 subjects in EPIC• Higher in men than women• Higher in vegetarians – linked to fiber intake • Positively associated with BMI• Positively associated with vigorous exercise

in women, not as clear in men• Sanjoaquin et al. Pub Health Nutr 2004;7:77-

83.

Fecal bulking index (FBI)Fecal bulking index (FBI)

• Increase over baseline in rehydrated fecal weight induced by a food, as a percentage of the increase induced by an equal weight of wheat bran reference

• Fecal weight required for health – 200 g/day (Spiller 1993)

• Fecal weight/g wheat bran fiber - 5.11 g (Cummings 1993)

• Monro. Eur J Clin Nutr 2004;58:32-39

Average increase in fecal weight per gram fiber fed Average increase in fecal weight per gram fiber fed

• Wheat 5.4 g/g fiber fed• Oats 3.4 g/g fiber fed• Corn 3.3 g/g fiber fed• Legumes 2.2 g/g fiber fed• Pectin 1.2 g/g fiber fed• Cummings JH. 1993. CRC Handbook of

Dietary Fiber in Human Nutrition

Inulin and stool weightInulin and stool weight

• 12 healthy males consumed a controlled diet for 3 weeks with and without 20 grams of inulin/day

• Significant increase in flatulence• No differences in stool weight – 20

g/day inulin increased daily stool weight less than 20 g/day

• Slavin & Feirtag. Food Funct 2011 2:72.

Limitations to fecal samplesLimitations to fecal samples

• Not practical in epidemiological studies• No accepted standard

– Stool weight– Stool chemistry– Microflora – methods, what is best– Stool frequency (easy to collect)– Quality of life (used in IBS trials)

Gut health and fiberGut health and fiber

• Central question: – Does dietary fiber from increase stool weight and

speed gastrointestinal transit?

– Generally accepted– Not supported by evidence-based review:

• Dietary Reference Intakes for fiber are based on epidemiologic studies on protection against CVD

• J Am Diet Assoc (2008) evidence-based review – fiber and laxation given a low score – Grade III - Fair

Gut health and fiberGut health and fiber

• Confounding Issues: – Many other diet factors affect laxation– Confounding effects of laxative use – Difficult to collect stool samples in these settings

• Wet and dry stool weight, transit time, fecal chemistry, microflora, how long to collect

– Are subjective measures enough support?• Frequency, ease of elimination, bloating, flatulence

Side effects of gut fermentationSide effects of gut fermentation

Gastrointestinal effects• Abdominal pain/cramps• Bloating• Colic• Distention• Flatulence/borborygmi• Laxation

– Frequency increased– Consistency more watery

• Diarrhea Livesey, Brit J Nutr 2001;85(Suppl 1):S7-S16

Subjective methodsSubjective methods

• Symptom questionnaires where subjects report the occurrence and severity or intensity of GI symptoms and frequency and consistency of bowel movements

• Need to define bloating, diarrhea, flatulence and describe rating scale

• Vary greatly among studies/subjects

Summary of tolerance dataSummary of tolerance data

• Dietary fiber – up to 80 g/d in vegetarians – no UL• Polydextrose – 50 g• Resistant starch – 80 g• Fructo-oligosaccharides – 10-15 g – diarrhea at 40 g• Sugar alcohols

– No effect dose – 20-50 g– Laxation threshold – 50-70 g– Grabitske & Slavin. Cr Rev Food Sci Nutr 2009;

49:327-360.

DisclaimersDisclaimers

• Definitions of tolerance – how much intestinal gas is acceptable

• Adaptation• Habitual or background diet• Total exposure to low digestible

carbohydrates – dietary fiber, resistant starch, oligosaccharides, polyols, etc.

No recommended UL for fiber No recommended UL for fiber

• Occasional adverse GI symptoms are observed when humans consume some of the isolated or synthetic fibers (Grabitske and Slavin. J Am Diet Assoc 2008;108:1677)

• Due to the bulky nature of fiber in foods, excess consumption is likely to be self- limiting.

Dietary fiber and laxationDietary fiber and laxation

• Not all fibers are equally effective in increasing stool weight

• Wheat bran is most effective and fibers that are extensively fermented during gut transit time (inulin, pectin, etc) have little effect on stool weight

ConclusionConclusion

• Fermentation of fiber difficult to study in vivo• Fermentation rates of interest – since quick

fermentation causes unwanted GI side effects• Markers of fiber fermentation in vivo – breath

gases, SCFAs – not shown to reliably measure fiber fermentation

• In vitro models for fermentation need to be developed and optimized